The present invention relates to a control apparatus and a control method for an internal combustion engine that directly injects fuel into a combustion chamber.
Japanese Unexamined Patent Publication No. 4-187841 discloses a direct injection internal combustion engine. In the prior art engine, fuel is directly injected into a combustion chamber. Then, a spark plug ignites a mixture of air and fuel formed in the combustion chamber. The engine is operated in a fuel injection mode selected between an intake stroke injection mode, in which the fuel is injected during the intake stroke of the engine, and a compression stroke injection mode, in which the fuel is injected during the compression stroke of the engine. The fuel injection mode is switched in accordance with the running characteristics of the engine.
When the intake stroke injection mode is executed, the amount of fuel to be injected into the combustion chamber (fuel injection amount) is determined in accordance with the flow rate of intake air (intake flow rate) to the combustion chamber. The fuel injection amount is determined in accordance with the intake flow rate also when the compression stroke injection mode is executed while the engine is cold, or while the engine is not warm. However, the intake flow rate required for the execution of the compression stroke injection mode is greater than the intake flow rate required for the intake stroke injection mode.
As soon as the fuel injection mode is switched, a throttle valve of the engine operates such that the intake flow rate required by the selected fuel injection mode is made available. However, there is a time delay from when the throttle valve is operated to when the actual intake flow rate reaches the required value. Therefore, immediately after the fuel injection mode is switched, the actual intake flow rate and the fuel injection amount, which is determined in accordance with the intake flow rate, have not yet reached the levels required for the selected fuel injection mode. As a result, when the fuel injection mode is switched, the engine speed temporarily fluctuates.
For, example, when the fuel injection mode is switched from the intake stroke injection mode to the compression stroke injection mode while the engine is cold, the engine speed temporarily decreases. When the fuel injection mode is switched from the compression stroke injection mode to the intake stroke injection mode while the engine is cold, the engine speed temporarily increases.
The objective of the present invention is to provide a control apparatus and a control method for a direct injection engine that reduces fluctuations of engine speed caused when switching fuel injection modes.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a control apparatus for an engine is provided. The engine includes an injector that directly injects fuel into a combustion chamber and a valve for controlling the flow rate of intake air to the combustion chamber. The engine is operated in a fuel injection mode selected from a first injection mode, in which the injector injects fuel during the compression stroke of the engine, and a second injection mode, in which the injector injects fuel during the intake stroke of the engine. The intake flow rate appropriate for the first injection mode differs from the intake flow rate appropriate for the second injection mode. The apparatus includes a controller for controlling the injector and the valve. When one of the fuel injection modes is selected, the controller controls the valve such that the intake flow rate is appropriate for the selected fuel injection mode. When the engine is cold, the controller determines the amount of fuel to be injected by the injector in accordance with the actual intake flow rate. The controller controls the valve such that the intake flow rate is appropriate for the selected fuel injection mode before actually switching the fuel injection mode by a predetermined time period.
The present invention also provides a method for controlling an engine that directly injects fuel into a combustion chamber. The method includes operating the engine in a selected one of a plurality of fuel injection modes. The injection modes include a first injection mode, in which the fuel is injected during the compression stroke of the engine, and a second injection mode, in which the fuel is injected during the intake stroke of the engine. The intake flow rate appropriate for the first injection mode differs from the intake flow rate appropriate for the second injection mode. The method further includes adjusting the intake flow rate to be appropriate for the selected fuel injection mode and determining the amount of fuel injected into the combustion chamber in accordance with the actual intake flow rate when the engine is cold. When a different injection mode is selected, the injection mode is switched to the selected injection mode after a delay that is sufficient to allow the intake flow rate to reach a level appropriate for the selected fuel injection mode.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.